Wafer arrangement for gas sensor

ABSTRACT

A gas sensor includes a multi-wafer stack of a plurality of layers and a measurement chamber. The plurality of layers includes a first layer comprising a sensor element that has a microelectromechanical system (MEMS) membrane; and a second layer comprising an emitter element configured to emit electromagnetic radiation. The measurement chamber is interposed between the first layer and the second layer. The measurement chamber is configured to receive a measurement gas and further receive the electromagnetic radiation emitted by the emitter element as the electromagnetic radiation travels along a radiation path from a first end of the measurement chamber to a second end of the measurement chamber that is opposite to the first end.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 16/944,279 filed on Jul.31, 2020, which is a continuation of U.S. patent application Ser. No.16/456,361 filed on Jun. 28, 2019, which is a continuation of U.S.patent application Ser. No. 15/079,840 filed on Mar. 24, 2016, whichclaims priority to German Application No. 20 2015 002 315.0 filed onMar. 27, 2015, which are incorporated by reference as if fully setforth.

FIELD

The present disclosure describes a gas sensor which has a MEMS membrane.Exemplary embodiments disclose a PAS (photoacoustic sensor) module,which uses a MEMS (microelectromechanical system) multi-wafer concept.

BACKGROUND

A MEMS device, which is also referred to as a microelectromechanicalsystem, is often used as a sensor, for example as acceleration sensors,pressure sensors or sound wave sensors (microphone). All of these MEMSdevices have a movable element, for example a membrane or a cantilever,wherein the movement of the movable element, as is caused, for example,by a pressure change or acceleration, can be detected capacitively.Thus, a conventional variant of a MEMS device comprises a movableelectrode as a movable element and a stationary electrode lying oppositeto the movable electrode such that a change in the distance between thetwo electrodes (due to the movement of the movable element) can lead toa capacitive change.

Previous gas sensor systems use components with dimensions in themillimeter to centimeter range. The components, e.g. an infraredemitter, therefore have comparatively large thermal masses, as a resultof which high powers are required to operate the gas sensors. These makethe systems sluggish and only allow very small duty cycles. Therefore,options for a quick calibration and a quick measurement are restricted.

Therefore, there is a need to develop an improved concept for gassensors.

SUMMARY

Exemplary embodiments disclose a gas sensor which comprises a sensorelement, a measurement chamber and an emitter element. The sensorelement has a MEMS membrane, wherein the MEMS membrane is arranged in afirst substrate region. Furthermore, the measurement chamber is embodiedto receive a measurement gas. The emitter element is embodied to emitelectromagnetic radiation, wherein the electromagnetic radiation passesover a radiation path, which includes the measurement chamber proceedingfrom the emitter element. Moreover, the emitter element and the sensorelement are arranged in a stationary manner with respect to one another,i.e. they are mechanically connected to one another for example.

Advantageously, a combination of components manufactured using MEMStechnology is used for an e.g. miniaturized gas sensor. These componentscan be connected in a so-called wafer stack or substrate stack and canform an emitter and a sensor which, in turn, can be connected to oneanother. By way of example, the gas sensor can be a PAS sensor(photoacoustic sensor), which uses the photoacoustic effect.

The photoacoustic effect is a physical effect which optoacoustics makesuse of. It describes the conversion of light energy into acoustic energy(sound). If a propagation medium, e.g. a gas, is irradiated with light,some of the light energy is received (absorbed) by the medium andconverted into heat. As a result of thermal conduction, the energy isdistributed in the medium after a finite period of time and a minimallyincreased temperature sets in in the medium. There is, inter alia, anincrease in volume as a result of the heat supply. There is periodicheating and cooling if the medium is irradiated by a sequence of lightflashes or, in general, by pulses of electromagnetic radiation. Thisconstant change in volume expansion and reduction constitutes a sourceof sound. This can be body-borne sound in a solid body or normal soundin gas.

Advantages emerge as a result of the very small dimensions of theoverall sensor system, as a result of which very small thermal masses ofthe sensor system can be realized. As a result, the power intake isreduced and high switching speeds are rendered possible, from which avery large duty cycle and hence a long overall service life result.Likewise, a shorter measurement cycle of a measurement emerges from thehigher switching speeds, as a result of which more measurements can becarried out in the same period of time. The described gas sensortherefore meets the highest quality requirements and has an increasedservice life compared to conventional gas sensors.

Exemplary embodiments disclose a gas sensor which comprises a sensorelement, a measurement chamber and an emitter element. The sensorelement has a MEMS membrane and a reference chamber with a referencefluid situated therein, wherein the MEMS membrane is arranged in a firstsubstrate region and a cavity of the reference chamber is arranged in asecond substrate region. The first and second substrate regions arehermetically sealed from, and connected to, one another. The measurementchamber is embodied to receive a measurement gas. The emitter element isembodied to emit electromagnetic radiation, wherein the electromagneticradiation passes over a radiation path, which includes the measurementchamber and the reference chamber proceeding from the emitter element,wherein the measurement chamber is spatially separated from thereference chamber by a layer through which electromagnetic radiation canpass. Moreover, the emitter element and the sensor element aremechanically connected to one another. The embodiment with a referencechamber and reference gas present therein is advantageous since thepressure measurement takes place in the sealed and known referencevolume and hence a greater number of realization options are available.The background is a variable adjustment option for the MEMS membrane orthe sensor element to a complete or partial selectivity of themeasurement gas or an avoidance of cross sensitivities. Thus, the sensorelement only reacts to the absorption wavelength of the reference gas,provided that the reference gas is pure and no “interference gases” arepresent. If an interference gas is present, a cross sensitivity mayoccur where the absorption wavelengths of the reference gas (or of themeasurement gas) and of the interference gas overlap. By way of example,when measuring CO2, there is a cross sensitivity to moisture at awavelength of approximately 2.2 μm since the absorption bands of carbondioxide and water overlap there.

Exemplary embodiments show that the MEMS membrane is embodied to convertenergy of the electromagnetic radiation, present in the referencechamber, into an output signal. By way of example, the conversion iscarried out by virtue of the MEMS membrane being embodied to have adeflection which is dependent on the energy of the electromagneticradiation present. This is advantageous since the electromagneticradiation excites the reference fluid to a larger vibration and hencethe increased particle movement or an increased pressure in thereference chamber can be measured by the MEMS membrane or a sensorformed with the MEMS membrane.

In accordance with exemplary embodiments, the emitter element isembodied to emit the electromagnetic radiation in a pulsating mannerwith a frequency that is typically greater than 0.1 Hz or greater than0.5 Hz or greater than 1 Hz. This is advantageous since this thereforeallows an increased number of measurements to be carried out within thesame period of time. Furthermore, the measurement density e.g. in thecase of continuous measurements is thus increased, with a change in ameasurement gas in the measurement chamber therefore being detected morequickly.

Exemplary embodiments furthermore disclose the emitter element, whichcomprises a first and a second substrate region, wherein the firstsubstrate region has an emitter unit, which is embodied to emit theelectromagnetic radiation. The second substrate region has a cavity,which is embodied to minimize a thermal mass of the emitter element.This is advantageous as the already described quick switching times ofthe emitter element can therefore be achieved. Furthermore, anunnecessary heating of the sensor is reduced. A heating of the sensorcan lead to quicker degradation. A cooling of the gas sensor fordissipating excessive heat can therefore have smaller dimensions or becompletely removed.

Moreover, the sensor or the MEMS membrane can preferably lie outside adirect beam path of the electromagnetic radiation emitted by the emitterin order to reduce heating of the MEMS membrane by the directelectromagnetic radiation. A further exemplary embodiment discloses thegas sensor with a shadow mask, which is arranged in the radiation path,wherein the shadow mask is embodied to reduce direct electromagneticradiation onto the MEMS membrane from the emitter element. This isadvantageous as this therefore delays a degradation of the MEMS membranesince a substantially smaller part of the latter is exposed to theelectromagnetic radiation. Furthermore, a pressure equalization chambersituated behind the MEMS membrane is heated less strongly by theelectromagnetic radiation, as a result of which a sensitivity oraccuracy of the gas sensor is ensured over a longer period of time.

In accordance with further exemplary embodiments, the emitter elementand the sensor element are arranged in a projection plane extendinglaterally with respect to the emitter element and the sensor element.Here, the emitter element and the sensor element are arranged in ahousing which is embodied to reflect the electromagnetic radiation fromthe emitter element onto the sensor element. This arrangement isadvantageous since it is therefore possible to realize extremely flatgas sensors. Here, the measurement chamber of the gas sensor can beembodied as a cavity in the housing. Furthermore, it is advantageous toembody the first substrate region of the sensor element and a firstsubstrate region of the emitter element on the same substrate and/or toembody the second substrate region of the sensor element and a secondsubstrate region of the emitter element on the same substrate. This isadvantageous since this allows manufacturing steps relating to the samesubstrate plane in the sensor element and in the emitter element to becarried out together in one manufacturing step. Hence, a production ofthese gas sensors is simplified, as a result of which an increase in theproductivity is achieved.

In accordance with a further exemplary embodiment, the emitter elementand the sensor element are arranged in a projection plane extending inthe thickness direction with respect to the emitter element and thesensor element, wherein the second substrate region of the sensorelement is connected to the emitter element in a hermetically sealedmanner. This is advantageous as a minimum overall size of the gas sensoris therefore obtained. Here, a cavity in the emitter element and/or acavity between the emitter element and the sensor element can form themeasurement chamber. If the measurement chamber is integrated into theemitter element, the gas sensor has a minimum height in the thicknessdirection. If the cavity is embodied between the emitter element and thesensor element, the emitter element can be spatially separated from themeasurement gas, as result of which a possibly increased degradation ofthe sensor element by the measurement gas is avoided.

Exemplary embodiments furthermore disclose the gas sensor, in whichcontacts of the emitter element and of the sensor element are guided bymeans of a through semiconductor via (TSV) within the emitter elementand the sensor element to a common substrate plane and embodied at amain surface region of the gas sensor that is accessible from outside.This is advantageous since contacts of the gas sensor are therefore onlyembodied at one position of the gas sensor and therefore simplifycontacting.

Exemplary embodiments furthermore describe that contacts of the emitterelement and the sensor element are embodied laterally at a surfaceregion of the emitter element and the sensor unit, wherein a printedcircuit board is arranged parallel to a thickness direction of theemitter element and the sensor element and contacts the laterallyembodied contacts. This is advantageous since, for example, the gassensor can be arranged on the printed circuit board without furthercontacting materials and can be connected electrically to the latter.

In accordance with further exemplary embodiments, the emitter elementhas an emitter unit for emitting the electromagnetic radiation, whichemitter unit is an infrared emitter.

Further exemplary embodiments disclose the gas sensor, wherein the MEMSmembrane forms a micromechanical capacitive sensor. By way of example,the micromechanical capacitive sensor is a microphone.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are explained below, withreference being made to the attached drawings. In detail:

FIG. 1A shows a schematic side view of a gas sensor;

FIG. 1B shows a schematic side view of a gas sensor with a referencechamber;

FIG. 1C shows a schematic side view of a gas sensor in an arrangement ofthe emitter element and the sensor element deviating from the one inFIG. 1A;

FIG. 2A shows, in a cross section, a schematic illustration of anemitter element and a sensor element which are used in a gas sensor;

FIG. 2B shows a schematic illustration of an emitter element and asensor element which are used in a gas sensor, in accordance with anexemplary embodiment with a reference chamber;

FIG. 2C shows a schematic illustration of an emitter element and asensor element which are used in a gas sensor, in accordance with afurther exemplary embodiment;

FIG. 2D shows a schematic illustration of an emitter element and asensor element in an alternative exemplary embodiment with a referencechamber;

FIG. 2E shows a schematic illustration of an emitter element with aperforated counter electrode lying at the top;

FIG. 3 shows a schematic illustration of a gas sensor in a housing,wherein the emitter element and the sensor element are arranged next toone another;

FIG. 4A shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment, wherein the emitter element and thesensor element are stacked in the thickness direction;

FIG. 4B shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment with a reference chamber, wherein theemitter element and the sensor element are stacked in the thicknessdirection;

FIG. 4C shows a schematic illustration of a gas sensor with a referencechamber in an exemplary embodiment deviating from the one in FIG. 4B;

FIG. 5A shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment, with exemplary contacting of the gassensor;

FIG. 5B shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment with a reference chamber and withexemplary contacting of the gas sensor;

FIG. 5C shows a schematic illustration of a gas sensor in accordancewith a deviating exemplary embodiment with a reference chamber and withexemplary contacting of the gas sensor;

FIG. 5D shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment with deviating contacting of the gassensor.

DETAILED DESCRIPTION

In the following description of the figures, identical elements orelements with identical effect are provided with the same referencesigns, and so the description thereof is interchangeable in thedifferent exemplary embodiments.

FIG. 1A shows a gas sensor 5 with a sensor element 10, a measurementchamber 15 and an emitter element 20. The sensor element 10 has a MEMSmembrane 25, which is arranged in a first substrate region 40. Themeasurement chamber 15 is embodied to receive a measurement gas 50.Moreover, the sensor element 10 and the emitter element 20 can have ahermetically sealed connection in accordance with one exemplaryembodiment such that a hermetically sealed measurement chamber 15 isformed. This can increase the service life of the emitter element 20 orof the whole gas sensor since the latter is operated in a protectiveatmosphere. The same effect can also be achieved by a housing whichsurrounds the gas sensor. By way of example, the MEMS membrane can forma micromechanical capacitive sensor such as e.g. a microphone. Themicromechanical capacitive sensor is embodied to measure a deflection ofthe MEMS membrane in relation to a counter electrode (not shown here)capacitively.

Furthermore, FIG. 1A shows the emitter element 20, e.g. an infraredemitter, which may be realized as a MEMS element, which is embodied toemit electromagnetic radiation 55. The electromagnetic radiation 55passes over a radiation path 60, which includes the measurement chamber15 proceeding from the emitter element 20. Furthermore, the emitterelement 20 and the sensor element 10 are mechanically interconnected.The hermetically sealed connection is optional since the confinement ofthe measurement gas in a volume can also be achieved, for example, byway of a housing shown in FIG. 3 . Furthermore, there are also exemplaryembodiments in which the confinement of the measurement gas is optional.If the measurement gas is not present in a sealed volume, it isadvantageous to embody the measurement gas or the measurement volume asan acoustic high-pass filter so that the photoacoustic signals of themeasurement gas act on the MEMS membrane and scattering into the freespace is avoided. Therefore, the acoustic high-pass filter rendersperforming continuous measurements in the case of a measurement gas infree space possible. In other words, a pressure change in an unsealedvolume or reference volume of the detector can be measured if the sensorelement is embodied as a high-pass filter. Therefore, continuousmeasurements are possible, for example with a sealed detector or with anembodiment of the detector as a high-pass filter. The describedexemplary embodiment constitutes a gas sensor which can be constructedto be very small since it is realized in the absence of the exemplaryembodiments with a reference chamber described below.

An improved measurement result can be achieved by separating the heattransfer from the emitter element 20 via the measurement gas 50. Inother words, it may be advantageous if heating of the measurement gas 50or an effect of a pressure change on the MEMS membrane due to theheating of the measurement gas is avoided. This can be achieved byclosing off the emitter element or the sensor element. If the emitterelement or the emitter is embodied in a closed-off manner, by a vacuumor an inert protective gas between an emitter unit, for example aheating wire, can be arranged, which allows infrared radiation from theemitter element to the measurement gas, but prevents or at least reducesheat propagation. If the sensor element has a closed-off embodiment andthe emitter element 20 is open, the measurement gas is heated but apressure change does not act on the MEMS membrane. In both cases, aphotoacoustic signal can be measured free from a superposition of anexpansion of the measurement gas caused by the heating. It is likewisepossible to hermetically seal the emitter element and the sensor elementin each case.

The described combinations of emitter element and sensor element can besubsumed under the term multi-wafer concept. It enables any combinationof sealed and open elements, which in each case form an open or closedemitter or sensor element. Likewise, the measurement chamber canoptionally also be embodied in a closed volume. The combinations of theelements may be arbitrary. Moreover, the emitter element and/or thesensor element can be embodied, per se, from layers of differentsubstrate elements or wafers. An exemplary realization is described inrelation to FIG. 2A-2E.

FIG. 1B shows a gas sensor 5 with a sensor element 10, a measurementchamber 15 and an emitter element 20. The sensor element 10 has a MEMSmembrane 25 and a reference chamber 30 with a reference fluid 35situated therein. The MEMS membrane 25 is arranged in a first substrateregion 40 and a cavity of the reference chamber 30 is arranged in asecond substrate region 45. The measurement chamber 15 is embodied toreceive a measurement gas 50. Furthermore, the first and secondsubstrate regions 40, 45, which form the sensor element 10, arehermetically sealed from, and connected to, one another. Moreover, thesensor element 10 and the emitter element 20 can also have ahermetically sealed connection in accordance with one exemplaryembodiment such that a hermetically sealed measurement chamber 15 isformed. This can increase the service life of the emitter element 20.

Furthermore, FIG. 1B shows the emitter element 20 which is embodied toemit electromagnetic radiation 55. The electromagnetic radiation 55passes over a radiation path 60, which includes the measurement chamber15 and the reference chamber 30 proceeding from the emitter element 20.In order to spatially separate the measurement gas and the referencefluid from one another, a layer 65 through which electromagneticradiation 55 can pass is arranged between the reference chamber 30 andthe measurement chamber 15. Furthermore, the emitter element 20 and thesensor element 10 are mechanically connected to one another.

By way of example, the reference fluid is a gas mixture which comprisesa gas to be detected, e.g. CO₂ (carbon dioxide), CO (carbon monoxide),NO_(x) (nitrogen oxide) etc., and optionally a buffer gas. By way ofexample, the buffer gas serves as a further reference gas by virtue ofit extending the selectivity of the reference cell to a gas mixture or afurther gas. Thus, in addition to the gas to be detected, one or morefurther gases may be present in the reference chamber such that the gassensor reacts sensitively to a measurement gas which comprises the gasespresent in the reference gas. Furthermore, it is possible to introducemoisture into the reference chamber in order to determine a moisturecontent of the measurement gas. Expressed differently, it serves as anelement for modifying or optimizing the transmission path, wherein thetransmission path has the following steps. Proceeding from a temperatureor electromagnetic radiation of the emitter element 20, a pressurechange is generated in the reference chamber 30, wherein the pressurechange depends on the absorption of the electromagnetic radiation by themeasurement gas (e.g. in an inversely proportional manner). The pressurechange in the reference chamber can be measured by a deflection of themembrane in the sensor element 10. Furthermore, a sensitivity of the gassensor 5 or of the sensor element 10 can be adjusted by way of thebuffer gas such that the desired sensitivity is obtained in the case ofan expected oversteer or understeer of the MEMS membrane, for example bya reduction or increase in the buffer gas component.

Exemplary embodiments show the MEMS membrane 25, which is embodied toconvert energy of the electromagnetic radiation present in the referencechamber 30 into an output signal. The output signal can be generated onthe basis of a deflection which is dependent on the energy of theelectromagnetic radiation present. Accordingly, the MEMS membrane canform, for example, a micromechanical capacitive sensor such as e.g. amicrophone. The micromechanical capacitive sensor is embodied to measurea deflection of the MEMS membrane, for example capacitively, in relationto a counter electrode (not shown here).

FIG. 1C shows a schematic illustration of the gas sensor 5 with anarrangement of the emitter element and the sensor element deviating fromthe one in FIG. 1A. FIG. 1C shows an arrangement of the emitter element20 and the sensor element 10 along a projection line which extendslaterally through the emitter element and the sensor element. Here, theemitter element 20 and the sensor element 10 are connected to oneanother at a laterally extending main surface region 70. In accordancewith one exemplary embodiment, the radiation path 60 of theelectromagnetic radiation 55 passes through the measurement gas 50adjoining the emitter element 20 and the sensor element 10. Thecurvature or deflection of the electromagnetic radiation 55 from themain surface region 75 of the emitter element, at which theelectromagnetic radiation is emitted, onto the main surface region 80 ofthe sensor element, at which the electromagnetic radiation enters intothe sensor element, is brought about, for example, by way of reflectingelements (not shown here), such as e.g. the inner side of a housingwhich has a reflecting action for the electromagnetic radiation 55. FIG.3 shows an exemplary embodiment of this arrangement.

FIG. 2A shows a schematic illustration of the emitter element 20 and ofthe sensor element 10, which are used in a gas sensor in accordance withexemplary embodiments. In addition to the first substrate region and thesecond substrate region 45, the sensor element 10 can additionally havea third substrate region 85 which, in the exemplary embodiment shownhere, constitutes a boundary element of the sensor element 10.Analogously to the sensor element, the emitter element can also have afirst substrate region 90 and a second substrate region 95, wherein thefirst substrate region 90 has an emitter unit 100 which is embodied toemit the electromagnetic radiation 55. By way of example, the emitterunit 100 is an infrared emitter (IR emitter), which can be realized by ameandering arrangement of a heating wire. The second substrate region 95of the emitter element has a cavity 105, which is embodied to minimize athermal mass of the emitter element. In accordance with furtherexemplary embodiments, the second substrate region 55 of the emitterelement can have a material transmissive to the electromagneticradiation 55 in addition or as an alternative to the cavity 105.Furthermore, the emitter element 20 also has a third substrate region110, which forms a boundary element of the emitter element. Moreover, itshould be noted that the exemplary embodiments described with respect toFIGS. 2B-2D can also be applied to FIG. 2A.

In accordance with this arrangement, the emitter element 20 can form ablack body. An ideal black body completely absorbs incidentelectromagnetic radiation of any wavelength and re-emits the receivedenergy as electromagnetic radiation with a characteristic spectrum thatonly depends on the temperature.

FIG. 2B shows a schematic illustration of the emitter element 20 and ofthe sensor element 10, which are used in a gas sensor in accordance withexemplary embodiments. In addition to the first substrate region and thesecond substrate region 45, the sensor element 10 can additionally havea third substrate region 85 which, in the exemplary embodiment shownhere, constitutes a boundary element of the sensor element 10.Analogously to the sensor element, the emitter element can also have afirst substrate region 90 and a second substrate region 95, wherein thefirst substrate region 90 has an emitter unit 100 which is embodied toemit the electromagnetic radiation 55. By way of example, the emitterunit 100 is an infrared emitter (IR emitter), which can be realized by ameandering arrangement of a heating wire. The second substrate region 95of the emitter element has a cavity 105, which is embodied to minimize athermal mass of the emitter element. In accordance with furtherexemplary embodiments, the second substrate region 95 of the emitterelement can have a material transmissive to the electromagneticradiation 55 in addition or as an alternative to the cavity 105.Furthermore, the emitter element 20 also has a third substrate region110, which forms a boundary element of the emitter element.

In accordance with this arrangement, the emitter element 20 can form ablack body. An ideal black body completely absorbs incidentelectromagnetic radiation of any wavelength and re-emits the receivedenergy as electromagnetic radiation with a characteristic spectrum thatonly depends on the temperature.

Analogous to the layer 65 of the second substrate region of the sensorelement transmissive to the electromagnetic radiation, the secondsubstrate region 95 of the emitter element can likewise have a layer 115transmissive to the electromagnetic radiation in accordance withexemplary embodiments, wherein FIG. 4B shows an exemplary embodiment inwhich the layers 65 and 115 form the measurement chamber. Furthermore,the layer 115 can optionally have an opening 120. The opening 120 canprovide access to the cavity 105 for the reference fluid such that anabsorption path for the temperature radiation emitted by the emitterelement is increased. Furthermore, cavities are optionally formed in thefirst and third substrate regions of the emitter element, which cavitiesare embodied in each case to reduce a thermal mass of the sensor elementand to form a hollow space 125 for realizing the black body. Optionally,the hollow spaces 125 and 105 can be filled with a buffer gas, which isembodied to minimize a degradation of the emitter unit and/or to improvea quality of the emitter element. In this exemplary embodiment, it isadvantageous to embody the layer 115 without an opening 120 in order toseparate the buffer gas from the measurement gas.

Furthermore, arranging a shadow mask 130 in the radiation path 60 (notplotted here) is likewise optional, said shadow mask being embodied toreduce direct electromagnetic radiation from the emitter element to theMEMS membrane 25. However, the shadow mask permits the entry of theelectromagnetic radiation into the reference chamber. To this end, theshadow mask, for example, only covers part of the layer of the sensorelement transmissive to the electromagnetic radiation or, in accordancewith the exemplary embodiment described in FIG. 2E, it is arranged infront of the MEMS membrane within the reference chamber. Expresseddifferently, an optional shadow mask does not shadow the referencechamber 30 but only the membrane 25. Advantageously, the layer 65transmissive to the electromagnetic radiation, e.g. a glass pane, iscoated or blackened to this end in a region of a projection line whichextends to the membrane 25 in the thickness direction. Therefore, theelectromagnetic radiation 55 can excite the reference fluid in thereference chamber 30 through the regions adjacent to the shadow mask130, but the direct electromagnetic irradiation of the MEMS membrane 25is significantly reduced. As a result, the MEMS membrane heats up to asignificantly lesser extent, as a result of which wear-and-tear or anerror signal of same, caused by a heating and a deflection of themembrane caused thereby (comparable to the effect in a bimetal), isreduced.

Furthermore, the shadow mask 130 reduces the heating of a pressureequalization chamber 135. The pressure equalization chamber 135 isconnected to the reference chamber 30 by way of an opening 140 in theMEMS membrane 25. There is a slow gas exchange between the referencechamber and the pressure equalization chamber through the opening suchthat changing pressures in the reference chamber and the pressureequalization chamber are equalized over a relatively long period of timeand pre-tensioning of the MEMS membrane 25, which changes over the longperiod of time, is avoided. Quick pressure changes cannot be equalizedby way of the opening 140, and so the MEMS membrane 25 or the sensorelement 10 is able to measure the quick changes. It should be noted thatthe shadow mask 130 is optional in all exemplary embodiments shown, evenif same as plotted in the associated drawings.

Advantageously, the substrate regions of the sensor element and of theemitter element can contain silicon. Therefore, the same substrateregions of the sensor element and of the emitter element can be producedin a common MEMS production method within the scope of one productionmethod. In accordance with exemplary embodiments, the substrate regionsproduced separately are stacked and arranged as a wafer stack orsubstrate stack. In order to fasten the substrate regions, same can beconnected to one another, for example by means of anodic bonding orglass frit bonding, such that connection elements depending on themethod are formed between the substrate regions (e.g. wafers). Thesecond substrate regions of the sensor element 45 or of the emitterelement 95 (top layers) or the first substrate regions of the sensorelement 85 or of the emitter element 110 (bottom layers) can also beembodied as a glass wafer or have a glass component, for example in theform of a window.

In the shown arrangement, the sensor element 10 and the emitter element20 can be operated laterally next to one another, for example within ahousing, as a pressure sensor (cf. FIG. 3 ) or, alternatively, besingulated by way of saw marks 145.

FIG. 2C shows the exemplary embodiment described in FIG. 2B, whereinFIG. 2B has been complemented by the spacers 160. If the sensor element10 and the emitter element 20 are assembled as shown in FIG. 4 , themeasurement chamber can be embodied between the spacers. The emitterelement and the MEMS membrane are sealed in the shown exemplaryembodiment and do not come into contact with the measurement gas. Inaddition to the arrangement of the spacer on the sensor element, shownhere, the spacer can also be arranged on the emitter element.Furthermore, FIG. 2C shows a sealed layer 115, i.e. the layer 115 doesnot have an opening 120. Therefore, the measurement gas does not comeinto contact with the emitter 110. Moreover, a protective gas can beintroduced into the cavities 105 and/or 125 in order to reduce adegradation of the emitter.

FIG. 2D shows a schematic illustration of the sensor element 10 and ofthe emitter element 20 in accordance with an exemplary embodimentdeviating from the one in FIG. 2B. The exemplary embodiment shows thesecond substrate regions 45, 95 of the sensor element and emitterelement, which contain e.g. glass or silicon dioxide (SiO₂). This can bea structured glass wafer. It can likewise be connected to the substrateregions lying therebelow by means of anodic bonding at the firstsubstrate region 40 or 90. Furthermore, the cavity 105 in this exemplaryembodiment is at least part of the measurement chamber 15, into which ameasurement gas to be measured can be introduced. The measurementchamber 15 therefore lies at least in part in the emitter element 20.

FIG. 2E shows, in a cross section, a schematic illustration of thesensor element 10 and of the emitter element 20. The exemplaryembodiment shows an unsealed emitter element 20 and a sealed sensorelement 10. An infrared window 65 can be arranged over the secondsubstrate region 45 of the sensor element in order to seal the sensorelement 10 in a gastight manner. As already described above, it can beconnected to the first substrate region 40 by way of anodic bonding orany other suitable method. Furthermore, the sensor element has aperforated counter electrode 180 lying on the top. The latter cancomprise polysilicon, a metal, layers made of a dielectric e.g. SiN(silicon nitride) and a metal or a combination of the aforementionedmaterials. Preferably, use can be made here of a material which reflectsinfrared radiation (e.g. a metal). This can be realized by a metalized,perforated counter electrode. The counter electrode 180 forms afunctional counterpart to the MEMS membrane in order to form amicromechanical capacitive sensor, for example a microphone.Furthermore, the counter electrode 180 has a perforated embodiment inthe shown exemplary embodiment and therefore simultaneously fulfills thetask of a shadow mask. The lower the degree of perforation is embodied,the better the shadowing of the MEMS membrane 25 is. The counterelectrode can be fastened to a holding structure 175, which e.g.contains an oxide.

Furthermore, it is also possible only to seal the emitter element oremitter in a gastight manner. The sensor element remains open. A heatingof the measurement gas can thus be avoided, for example, by aconfinement of the emitter unit, for example of the heating wire, suchthat only the emitted infrared radiation is incident on the measurementgas and it causes the photoacoustic signal there, which signal can bemeasured by the sensor element. An expansion of the measurement gas as aresult of the heating, which is transferred to the MEMS membrane and issuperimposed on the photoacoustic signal, is therefore avoided.

FIG. 3 shows, in a schematic illustration, the arrangement, alreadyshown in FIG. 2D, of emitter element and sensor element in a housing150. The housing 150 has entrance and exit openings 155 a, b for themeasurement gas 50, through which the measurement gas 50 can enter intothe housing 150. By way of example, the housing 150 is an SMD (surfacemounted device) housing. Furthermore, the housing 150 has main surfaceregions on a side facing the sensor element and the emitter element,which main surface regions are embodied to reflect the electromagneticradiation 55 from the emitter element 20 onto the sensor element 10.

FIG. 4A shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment, wherein the emitter element and thesensor element are stacked in the thickness direction. The gas sensor isbased on the arrangement of the emitter element and the sensor elementalready shown in FIG. 2A. In order to obtain the exemplary embodimentshown in FIG. 4A, the sensor element can be separated from the emitterelement along the saw mark 145 shown in FIG. 2A and be stacked on outermain surface regions of the respective second substrate regions. Aspacer 160 can preferably be arranged between the sensor element and theemitter element, which spacer forms the measurement chamber 15 betweenthe sensor element and the emitter element. The spacer 160 can be e.g. aconnecting element, which was generated by connecting the adjoiningsubstrate regions. Furthermore, the spacers can comprise a semiconductormaterial (e.g. silicon) or glass. Measurement gas can be introduced intothe measurement chamber through an opening in the spacer 160. Takinginto account the absence of a reference chamber, the further exemplaryembodiments described with reference to the following FIGS. can also beapplied to the exemplary embodiment shown here in an analogous manner.

FIG. 4B shows a schematic illustration of a gas sensor in accordancewith one exemplary embodiment, wherein the emitter element and thesensor element are stacked in the thickness direction. The gas sensor isbased on the arrangement of the emitter element and the sensor elementalready shown in FIG. 2B. In order to obtain the exemplary embodimentshown in FIG. 4B, the sensor element can be separated from the emitterelement along the saw mark 145 shown in FIG. 2B and stacked on outermain surface regions of the respective second substrate regions. Aspacer 160 can preferably be arranged between the sensor element and theemitter element, which spacer forms the measurement chamber 15 betweenthe sensor element and the emitter element. The spacer 160 can be e.g. aconnecting element, which was generated by connecting the adjoiningsubstrate regions. Furthermore, the spacers can comprise a semiconductormaterial (e.g. silicon) or glass. Measurement gas can be introduced intothe measurement chamber through an opening in the spacer 160.

As already described, the sensor element and the emitter element can beconnected together with the spacer, for example by means of anodicbonding or a different bonding method. Furthermore, the spacer itselfcan also be embodied by a suitable connection material, for example of aconnection layer in the case of a bonding method with an intermediatelayer, as is used, for example, in eutectic bonding, glass frit bondingor adhesive bonding. This arrangement is advantageous since themeasurement gas 50 (not shown here) does not come into contact witheither the emitter element or the sensor element in the measurementchamber 15, but rather it is present in a defined region between thesensor element and the emitter element, which is delimited by the spacer160 and the layers 65 and 115 transmissive to the electromagneticradiation. This avoids a potential contamination and an accelerateddegradation, caused by the measurement gas, of the sensor element andthe emitter element. In other words, the sensor element and the emitterelement themselves are closed off or sealed.

Exemplary embodiments show the gas sensor in a manner sensitive tocarbon dioxide (CO₂). By way of example, a carbon dioxide concentrationin the measurement gas can have 1000 ppm (parts per million). Areference fluid, which e.g. comprises 50 to 100% carbon dioxide and,optionally, a buffer gas component, can be present in the referencechamber and the pressure equalization chamber 30, 135. Optionally, thebuffer gas filling can also furthermore be present in the hollow space125. If the buffer gas is present adjacent to the emitter unit, i.e. inthe cavities 105 or 125, it can serve as an inert protective gas, i.e.slow down a degradation of the emitter. Nitrogen, argon or other heavygases, which prevent or at least slow down a surface modification, e.g.caused by a great heat of the emitter, can serve as a protective gas.

Alternatively, or in a complementary manner, the buffer gas at theemitter can also serve to filter the output radiation in order to(further) restrict comparatively broadband electromagnetic radiationfrom the emitter unit in its bandwidth such that more narrowbandelectromagnetic radiation is incident on the measurement gas. Thecomparatively broadband electromagnetic radiation at the emitter canhave a bandwidth of between 1 μm and 10 μm, with the filtered morenarrowband electromagnetic radiation for example having a bandwidth ofbetween 0.2 μm and 0.5 μm. In order to obtain the measurement accuracyof the gas sensor, it is advantageous that the buffer gas is free fromthe measurement gas to be determined. A filter function can also beachieved by a suitable separation of the emitter from the measurementchamber. By way of example, the layer 115 transmissive to theelectromagnetic radiation is therefore embodied as a filter element.This can be achieved by a special treatment of a glass pane or the useof a Fabry-Perot filter.

If the buffer gas is present adjacent to the MEMS membrane 25 or in thereference chamber, it can likewise fulfill the function of a protectivegas for the MEMS membrane. Alternatively, or in a complementary manner,it can also comprise as a reference the measurement gas to bedetermined, i.e., for example, the gas to be measured or the gas mixtureto be measured.

The exemplary embodiment shows the emitter element 20 and the sensorelement 10 arranged in a projection plane extending in the thicknessdirection to the emitter element and the sensor element, wherein thesecond substrate region of the sensor element is connected to theemitter element in a hermetically sealed manner. As already described,the spacer 160 can be inserted between the emitter element and thesensor element. The connection is advantageously brought about by meansof anodic bonding or another suitable method for connecting substrateregions. For the purposes of electrical contacting of the emitterelement and the sensor element, connectors of the sensor unit and theMEMS membrane, which e.g. can be contacted by way of contact pads, canbe embodied between the second and the third substrate regions orbetween the first and the second substrate regions of the sensor elementand the emitter element.

FIG. 4C shows a schematic illustration of the gas sensor 5, which isconstructed on the basis of the sensor element 10 and the emitterelement 20 of the exemplary embodiment shown in FIG. 2D. As alreadydescribed with respect to FIG. 4B, the sensor element and the emitterelement can be separated at the saw mark 145 such that the secondsubstrate region in the sensor element can be connected to the secondsubstrate region of the emitter element in such a way that a gas sensorthat is stacked in the thickness direction is created, which gas sensorcomprises six substrate regions in the shown exemplary embodiment. Theshown exemplary embodiment has a sealed microphone 25, whereas themeasurement chamber is integrated into the emitter element and is indirect contact with the emitter unit 100. This exemplary embodimentallows the smallest embodiment of the gas sensor 5 in the x-, y- andz-direction.

FIG. 5A shows a schematic illustration of the gas sensor 5 in accordancewith an exemplary embodiment without a reference chamber, which is basedon the exemplary embodiments shown in FIG. 2A and FIG. 4A. Theembodiment of the emitter element 20, in which the first substrateregion 90 is rotated by 180° about an axis extending laterally throughthe substrate region, deviates from the aforementioned exemplaryembodiments. The emitter unit 100 can be arranged superficially on thesubstrate region 90 and be exposed on the rear side, for example byetching, from an opposite main surface region. Hence, the emitterelement 20 can be operated on the rear side. The same principle is alsoapplicable to the MEMS membrane 25 in the first substrate region 40 ofthe sensor element, but this is not explicitly shown in the FIGS.

Furthermore, FIG. 5A shows contacting of the gas sensor 5 on a printedcircuit board (PCB) 165, which is explained in more detail on the basisof the following FIGS. Contacting in accordance with the TSV 170′described in FIG. 5B is likewise possible, just as the other exemplaryembodiments, which are described with respect to FIGS. 5B-5D, areapplicable to FIG. 5A.

FIG. 5B shows a schematic illustration of the gas sensor 5 in accordancewith an exemplary embodiment, which is based on the exemplaryembodiments shown in FIG. 2B and FIG. 4B. The embodiment of the emitterelement 20, in which the first substrate region 90 is rotated by 180°about an axis extending laterally through the substrate region, deviatesfrom the aforementioned exemplary embodiments. The emitter unit 100 canbe arranged superficially on the substrate region 90 and be exposed onthe rear side, for example by etching, from an opposite main surfaceregion. Hence, the emitter element 20 can be operated on the rear side.The same principle is also applicable to the MEMS membrane 25 in thefirst substrate region 40 of the sensor element, but this is notexplicitly shown in the FIGS.

Furthermore, FIG. 5B shows contacting of the gas sensor 5 on a printedcircuit board (PCB) 165. FIG. 5B shows the contacting by means ofcontact elements 170, for example by means of wires, which contactcontacts of the gas sensor 5, embodied toward the outside, with theconductor track 165. To this end, it is advantageous if the substrateplane 40 of the sensor element has a greater diameter than the substrateregions lying thereabove, as shown in FIG. 5B. Hence, contact structuresof the MEMS membrane 25 can be exposed at a main surface region of thefirst substrate region 40. The contacts of the emitter unit 100 arelikewise embodied at a main surface region and connected to theconductor track 165 by way of a contact element 170.

Alternatively, the connectors of the MEMS membrane 25 and of the emitterunit 100 can be guided in the interior of the substrate regions to theconductor track 165, for example by means of TSV 170′, and be contactedto said conductor track there.

It is likewise possible, but not shown, to guide e.g. the connector ofthe emitter unit 100 to a substrate plane of the MEMS membrane andundertake the contacting with both contacting elements 170 there.

FIG. 5C shows an exemplary embodiment, which is based on the exemplaryembodiments of FIGS. 2D and 4C. As already described in FIG. 5B, theemitter unit 100 is operated on the rear side. Furthermore, the etchedcavity of the substrate region 90 is embodied as a measurement chamber15. As already described with respect to FIG. 5B, FIG. 5C shows thecontacting of the printed circuit board 165 by means of the contactingelements 170.

FIG. 5D shows a further contacting option on the basis of the exemplaryembodiment from FIG. 4C. In accordance with this exemplary embodiment,contacts of the emitter element and the sensor element are embodiedlaterally on a main surface region of the emitter element and the sensorunit. The laterally embodied contacts are directly contacted with theprinted circuit board 165, which is arranged parallel to a thicknessdirection of the emitter element and the sensor element.

Although some aspects have been described in connection with a device,it goes without saying that these aspects also represent a descriptionof the corresponding method, such that a block or a component of adevice should also be understood as a corresponding method step or as afeature of a method step. Analogously to this, aspects described inconnection with or as a method step also represent a description of acorresponding block or detail or feature of a corresponding device.

The exemplary embodiments described above merely constitute anillustration of the principles of the present disclosure. It goeswithout saying that modifications and variations of the arrangements anddetails described herein will become apparent to other persons skilledin the art. Therefore, the intention is for the disclosure to berestricted only by the scope of protection of the following patentclaims, and not by the specific details presented herein on the basis ofthe description and the explanation of the exemplary embodiments.

What is claimed is:
 1. A gas sensor, comprising: a multi-wafer stack ofa plurality of layers, wherein the plurality of layers comprise: a firstlayer comprising a sensor element that has a microelectromechanicalsystem (MEMS) membrane; and a second layer comprising an emitter elementconfigured to emit electromagnetic radiation; and a measurement chamberinterposed between the first layer and the second layer, the measurementchamber being configured to receive a measurement gas and furtherreceive the electromagnetic radiation emitted by the emitter element asthe electromagnetic radiation travels along a radiation path from afirst end of the measurement chamber to a second end of the measurementchamber that is opposite to the first end.
 2. The gas sensor of claim 1,further comprising: a spacer structure defining the measurement chamber,wherein the spacer structure is mechanically coupled to the first layerat the first end of the measurement chamber and mechanically coupled tothe second layer at the second end of the measurement chamber.
 3. Thegas sensor of claim 2, wherein the spacer structure comprises at leastone of the plurality of layers of the multi-wafer stack.
 4. The gassensor of claim 1, wherein the plurality of layers further comprise: athird layer coupled to the first layer, wherein the first layercomprises a first cavity and the third layer comprises a second cavitythat is conjoined with the first cavity to form a pressure equalizingchamber, wherein the MEMS membrane is interposed between the measurementchamber and the pressure equalizing chamber.
 5. The gas sensor of claim4, wherein the pressure equalizing chamber is filled with a protectivegas.
 6. The gas sensor of claim 5, wherein the pressure equalizingchamber is sealed from the measurement chamber by the MEMS membrane. 7.The gas sensor of claim 4, wherein the MEMS membrane includes an openingthat connects the measurement chamber to the pressure equalizingchamber.
 8. The gas sensor of claim 1, wherein the plurality of layersfurther comprise: a third layer coupled to the second layer, wherein thesecond layer comprises a first cavity and the third layer comprises asecond cavity that is conjoined with the first cavity to form anenclosed hollow space.
 9. The gas sensor of claim 8, wherein the emitterelement is interposed between the measurement chamber and the enclosedhollow space.
 10. The gas sensor of claim 9, wherein the enclosed hollowspace is filled with a protective gas.
 11. The gas sensor of claim 1,wherein the plurality of layers further comprise: a third layer coupledto the first layer, wherein the first layer comprises a first cavity andthe third layer comprises a second cavity that is conjoined with thefirst cavity to form a pressure equalizing chamber, wherein the MEMSmembrane is interposed between the measurement chamber and the pressureequalizing chamber; and a fourth layer coupled to the second layer,wherein the second layer comprises a third cavity and the fourth layercomprises a fourth cavity that is conjoined with the third cavity toform an enclosed hollow space.
 12. The gas sensor of claim 11, whereinthe emitter element is interposed between the measurement chamber andthe enclosed hollow space.
 13. The gas sensor of claim 11, wherein thepressure equalizing chamber is sealed from the measurement chamber bythe MEMS membrane.
 14. The gas sensor of claim 11, wherein the MEMSmembrane includes an opening that connects the measurement chamber tothe pressure equalizing chamber.
 15. The gas sensor of claim 1, whereinthe measurement gas converts the electromagnetic radiation into aphotoacoustic signal, and the sensor element is configured to measurethe photoacoustic signal by detecting deflections of the MEMS membrane.16. The gas sensor of claim 1, wherein the MEMS membrane is configuredto have a deflection which is dependent on an energy of theelectromagnetic radiation.
 17. The gas sensor of claim 1, wherein theemitter element is configured to emit the electromagnetic radiation in apulsating manner with a frequency that is greater than 0.1 Hz.